1. Field of the Invention
The present invention relates to a structure of a thermal head applicable to output terminals such as printers, plotters, facsimiles and, more particularly, to patterns of electrodes for supplying heating signals to heating elements juxtaposed on a substrate.
2. Description of the Related Arts
A typical thermal head consists of a single relatively small-sized substrate having thereon a heating element, common electrodes, and individual electrodes. Through the electrodes, the heating element is supplied with electrical power to generate heat. Thermal paper or ink film is brought into contact with the heating element by way of a protective film.
On the contrary, there is a recently increasing demand for a large-sized thermal head for use in the output terminals such as a CAD system. The large-sized thermal head is preferably composed of a single thermal head substrate. In this case, however, the heating elements or electrodes free from any defect must be formed over a large area of the substrate. The manufacture of such large-sized substrate will therefore face a large technical difficulty, which is not practical.
In the prior art, therefore, a plurality of small-sized substrates were arranged to constitute a large-sized thermal head, as disclosed in Japanese Patent Laid-open Pub. No. 2-72967.
This will be described in detail with reference to FIGS. 1 and 2. FIG. 1 shows, by way of example, a single thermal head consisting of three transversely alternately arranged substrates including substrates 10a and 10c having, in plan view, a substantially trapezoidal shape, and a substrate 10b having an inverted trapezoidal shape. FIG. 2 is an enlarged view of the thermal head depicted in FIG. 1.
On the substrates 10a, 10b, and 10c, alternately arranged are comb-teeth-like common electrodes 12 and also comb-teeth-like individual electrodes 14 on which a belt-like heating element 16 extends. The heat is generated in the region of the heating element 16 intervening between the common electrodes 12 and the individual electrodes 14 adjoining to each other.
The edges of the substrates 10a, 10c and the substrate 10b adjoining to each other correspond to the oblique sides of the trapezoid of each substrate. The edges of the substrates 10a, 10c and 10b (hereinafter, referred to as side edge portions of the substrate) are formed at predetermined angles .theta.3 and .theta.3' making supplementary angles other than 90 degrees. Moreover, the substrates 10a and 10c are displaced from the substrate 10b by a distance L with respect to the paper feed direction to thereby prevent the heating element 16 from being discontinuous between the adjacent substrates.
At the right and left side edge portions of the substrate, the electrodes 12 and the individual electrodes 14 are both disposed to form a predetermined angle .theta.5' in a counterclockwise direction or a predetermined angle .theta.5 with respect to the direction orthogonal to the longitudinal direction of the substrate on which the heating element 16 is arranged. The angles of the electrode patterns .theta.5' and .theta.5 are substantially equal to the angles .theta.3' and .theta.3 of the side edge portions of the substrates so that the electrodes can be disposed for printing even at the side edge portions of the substrate having, in plan view, a substantially trapezoidal shape or an inverted trapezoidal shape.
In the case where a single thermal head is constituted of three alternately arranged substrates 10a, 10b, and 10c of a substantially trapezoidal shape or inverted trapezoidal shape having electrode patterns as shown in FIGS. 1 and 2, there is the inconvenience that at least two types of electrode patterns are required, that is, one for the trapezoidal substrates 10a and 10c, and the other for the inverted trapezoidal substrate 10b.
To this end, at least two systems of photo masks are needed for manufacturing three substrates to constitute a single thermal head, which will lead to increased development cost and development time. Furthermore, producing, cutting and assembling steps are necessary for two types of substrates, which will add to the number of steps.
Providing that a thermal head including a combination of two types of substrate having a trapezoidal shape and an inverted trapezoidal shape is formed with only one electrode pattern as in the substrates 10a and 10c, the positions where the common electrodes 12 are arranged on the substrates 10a and 10c will be where the paper arrives earlier than the heating element 16 with respect to the paper feed direction F, while the positions of the common electrodes 12 to be arranged on the substrate 10b will be where paper arrives later than the heating element 16, as shown in FIG. 3.
Therefore, when these substrates are employed to constitute a single thermal head, the positions of the common electrodes 12 and the individual electrodes 14 to be arranged with respect to the paper feed direction F will differ for each of the adjacent substrates and hence the direction of the data transfer will differ for each of the substrates, which results in complicated wiring and a complicated structure of the thermal head.